Plasmonics relies upon the coupling of light into free electron plasma in metals to create a wave of surface charge oscillation called plasmon. Plasmon is typically associated with a highly concentrated electromagnetic field, which is a key feature in many of its applications.
Plasmon can exist only at the surface of a metal or at the surface of any other material with negative dielectric permittivity (epsilon<0, within the operation spectral range), and it is often referred to as surface plasmon. Thus, the metal (or other material with epsilon<0, within the operation spectral range) is an essential component of any plasmonic device, and the optical properties of the metal used in a given plasmonic device will dictate the performance of the device. Metals have been characterized by large optical losses, limiting the performance of modern plasmonic devices.
Conventionally, gold and silver have been the metals of choice for plasmonic devices, due to having the lowest optical losses among metals. However, gold and silver are still not the best materials to fabricate and integrate into plasmonic devices because of several problems associated with their properties. First, their optical losses are small but not insignificant. In the visible range, the losses are relatively high for gold due to interband absorption. Additionally, gold and silver do not have optical properties that can be tailored, adjusted, or tuned to suit a particular application. Second, gold and silver are difficult to fabricate into ultra-thin films or nanostructures, which are often necessary in plasmonic devices. Moreover, patterning on the nanoscale level leads to additional optical loss in such metals. Third, silver and gold are not thermally stable at high temperatures, especially when nanostructured. Fourth, silver is not chemically stable and causes problems in many applications (e.g., in sensing). Fifth, neither metal is CMOS compatible, hence posing challenges in the integration of plasmonic devices with nanoelectronic CMOS devices.
The problems associated with gold and silver severely limit the development of plasmonics as a science into a technology. Hence, alternative plasmonic materials are essential to the further development of this technology.
U.S. Pat. No. 8,427,925, to Zhao et al. (hereafter “Zhao”) is one example of technology which employs the use of alternative plasmonic materials. The functionality of a given plasmonic material depends on nanoscale features such as the size of nanoparticles, the distance between them, etc. While Zhao discusses improvement of mechanical stability by adding elements such as TiN to gold, the present invention goes further by providing not only for mechanical stability, but also for the fact that the nanoelements keep their shape and optically perform stable over a longer period, a feature which goes beyond simple mechanical stability.